Comparative Chemical Composition, Antioxidant and Antimicrobial Activities of Three Thymus Species Essential Oils T. serpyllum, T. mastichina, T. × citriodorus

Abstract

This study compares the chemical composition, antioxidant, and antibacterial effects of essential oils (EOs) from three Thymus species collected in Morocco T. serpyllum, T. mastichina, and T. × citriodorus. The EOs were extracted by hydrodistillation using a Clevenger apparatus, then GC-MS was used to establish volatile profiles, and DPPH, ABTS, and FRAP were used to evaluate antioxidant performance. The agar diffusion and broth microdilution methods were used to assess antibacterial activity against representative gram-positive (Staphylococcus aureus, Bacillus subtilis, Streptococcus spp.) and gram-negative (Pseudomonas aeruginosa, Klebsiella pneumoniae) strains to determine minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values. Oxygenated monoterpenes were the most abundant class across species (71.61–90.67% of total identified area). T. mastichina exhibited a cineole/linalool chemotype (1,8-cineole 45.13%; linalool 5.64%), T. serpyllum displayed a cineole/terpinen-4-ol profile with moderate phenolics (1,8-cineole 36.12%; terpinen-4-ol 7.76%; thymol 6.02%), and T. × citriodorus was geraniol/citral type (geraniol 61.40%; neral 7.60%; geranial 9.04%). T. serpyllum was more potent as an antioxidant than T. × citriodorus and T. mastichina in all assays (P < 0.05), indicating assay concordance between radical-scavenging and reducing-power measures. The same general trend was observed in antibacterial activity: T. serpyllum generated the largest zones of inhibition and lowest MIC/MBC values, T. × citriodorus was intermediate, and T. mastichina was lower but still bactericidal (MBC/MIC ≤ 2). Gram-positive bacteria were more susceptible than gram-negative species. Comprehensive chemotaxonomic analysis (hierarchical clustering, PCA, k-means) of our samples with 83 literature samples representing 10 Thymus species from 18 countries revealed that essential oil composition clusters primarily by species identity rather than geographic origin, with our Moroccan samples grouping with their respective species globally rather than forming a regional cluster.